Nuclear magnetic resonance apparatus, systems, and methods

ABSTRACT

A method includes disposing a downhole tool having a magnet assembly into a wellbore. The method includes generating, using the magnet assembly, a magnetic polarization in a volume into a subterranean region about the wellbore. The method also includes emitting an excitation in the magnetic polarization in the volume in the subterranean region. The method includes detecting, by at least one antenna, a nuclear magnetic resonance response to the excitation of the volume in the subterranean region. The method also includes determining a property of the subterranean region based on the nuclear magnetic resonance response.

CLAIM FOR PRIORITY

This application claims priority to U.S. application Ser. No. 15/103,782filed on Jun. 10, 2016 (now allowed), which claims priority to PCTApplication No. PCT/US2016/017309 filed on Feb. 10, 2016, which claimspriority to U.S. Provisional Application Ser. No. 62/128,746, filed onMar. 5, 2015, wherein each of the three above-referenced applications inthe claim for priority are hereby incorporated herein by reference inits entirety.

BACKGROUND

In the field of logging (e.g. wireline logging, logging while drilling(LWD) and measurement while drilling (MWD)), nuclear magnetic resonance(NMR) tools have been used to explore geographic formations based on themagnetic interactions with subsurface material. Some downhole NMR toolsinclude a magnet assembly that produces a static magnetic field, and acoil assembly that generates radio frequency (RF) control signals anddetects magnetic resonance phenomena in the subsurface material.Properties of the subsurface material can be identified from thedetected phenomena.

When using NMR-based tools, it can be difficult to achieve asatisfactory signal-to-noise ratio (SNR) based on the naturalpolarization of the spins. In addition, the ex situ (i.e., outwardlooking) nature of downhole NMR tools means that the excitation fieldnaturally falls off in strength over distance from the tool. Thus, thefurther the antenna is from the sensitive volume, the lower theamplitude of the corresponding received signal.

DESCRIPTION OF DRAWINGS

FIG. 1A is a diagram of an example well system, according to variousembodiments.

FIG. 1B is a diagram of an example well system that includes an NMR toolin a wireline logging environment, according to various embodiments.

FIG. 1C is a diagram of an example well system that includes an NMR toolin a logging while drilling (LWD) environment, according to variousembodiments.

FIG. 2A is a diagram of an example downhole tool for obtaining NMR datafrom a subterranean region, according to various embodiments.

FIG. 2B is a diagram of another example downhole tool for obtaining NMRdata from a subterranean region, according to various embodiments.

FIG. 3A is a plot showing azimuthal selectivity for an example downholetool, according to various embodiments.

FIG. 3B is a diagram of another example downhole tool for obtaining NMRdata from a subterranean region, according to various embodiments.

FIG. 4A is a flowchart showing an example technique for obtaining NMRdata from a subterranean region, according to various embodiments.

FIG. 4B is a flowchart showing another example technique for obtainingNMR data from a subterranean region, according to various embodiments.

FIG. 5 is a schematic view of an NMR tool configuration, according tovarious embodiments.

FIG. 6 is a wireframe model view of the NMR tool configuration shown inFIG. 5.

FIG. 7 illustrates side, cut-away views of a portion of aradially-symmetric magnetic field profile, according to variousembodiments.

FIG. 8 illustrates side, cut-away views of a portion of a modeledradially-symmetric magnetic field profile, according to variousembodiments.

FIG. 9 illustrates side, cut-away views of a portion of a modeledradially-symmetric magnetic field profile, as pole magnets move awayfrom the center, according to various embodiments.

FIG. 10 illustrates side, cut-away views of a portion of a modeledradially-symmetric magnetic field profile, as pole magnets move awayfrom the center, at an operating frequency of 295 kHz, according tovarious embodiments.

FIG. 11 illustrates side, cut-away views of a portion of a modeledradially-symmetric magnetic field profile, as pole magnets move awayfrom the center, at an operating frequency of 716 kHz, according tovarious embodiments.

FIGS. 12-13 are schematic views of NMR tool configurations, according tovarious embodiments.

FIG. 14 illustrates a field profile of the pillow zone in the magneticfield created by the addition of magnetically permeable material to thetool configuration, according to various embodiments.

FIGS. 15-16 are schematic views of NMR tool configurations, according tovarious embodiments.

FIG. 17 illustrates symmetric field profiles of the magnetic fields thatcorrespond to the NMR tool configurations of FIGS. 15-16, according tovarious embodiments.

FIG. 18 is a schematic view of an NMR tool configuration, according tovarious embodiments.

FIG. 19 illustrates asymmetric field profiles of the magnetic fieldsthat correspond to the NMR tool configuration of FIG. 18, according tovarious embodiments.

FIG. 20 is a schematic view of an NMR tool configuration, according tovarious embodiments.

FIG. 21 is a block diagram of apparatus and systems according to variousembodiments.

FIG. 22 is a flow chart illustrating several methods according tovarious embodiments.

DETAILED DESCRIPTION

In some embodiments, an NMR-based tool can be constructed to providesubsurface data with an improved SNR. For example, a tool constructedaccording to various embodiments disclosed herein may includestrategically placed magnets, magnetically permeable material, andcopper to provide a higher SNR at a given distance into the formationthan conventional tools can offer.

Some examples include a magnet assembly to produce a magnetic field in avolume in a geological formation, the magnet assembly comprising acentral magnet having a first axial end and a second, opposite axialend; a first end piece magnet spaced apart from the first axial end ofthe central magnet; and a second end piece magnet spaced apart from thesecond axial end of the central magnet. The magnet assembly alsocomprises at least one shim magnet disposed between the central magnetand the first end piece magnet, the shim magnet disposed adjacent to orat least partially surrounded by a magnetically permeable material, theshim magnet to shape a static magnetic field sub-volume provided by thecentral magnet and the first end piece magnet. A downhole tool may beattached to the magnet assembly. These and many other embodiments willnow be described in detail.

FIG. 1A is a diagram of an example well system 100 a, according tovarious embodiments. The example well system 100 a includes an NMRlogging system 108 and a subterranean region 120 beneath the groundsurface 106. A well system can include additional or different featuresthat are not shown in FIG. 1A. For example, the well system 100 a mayinclude additional drilling system components, wireline logging systemcomponents, etc.

The subterranean region 120 can include all or part of one or moresubterranean formations or zones. The example subterranean region 120shown in FIG. 1A includes multiple subsurface layers 122 and a wellbore104 penetrated through the subsurface layers 122. The subsurface layers122 can include sedimentary layers, rock layers, sand layers, orcombinations of these and other types of subsurface layers. One or moreof the subsurface layers can contain fluids, such as brine, oil, gas,etc. Although the example wellbore 104 shown in FIG. 1A is a verticalwellbore, the NMR logging system 108 can be implemented in otherwellbore orientations. For example, the NMR logging system 108 may beadapted for horizontal wellbores, slanted wellbores, curved wellbores,vertical wellbores, or combinations of these.

The example NMR logging system 108 includes a logging tool 102, surfaceequipment 112, and a computing subsystem 110. In the example shown inFIG. 1A, the logging tool 102 is a downhole logging tool that operateswhile disposed in the wellbore 104. The example surface equipment 112shown in FIG. 1A operates at or above the surface 106, for example, nearthe well head 105, to control the logging tool 102 and possibly otherdownhole equipment or other components of the well system 100. Theexample computing subsystem 110 can receive and analyze logging datafrom the logging tool 102. An NMR logging system can include additionalor different features, and the features of an NMR logging system can bearranged and operated as represented in FIG. 1A or in another manner.

In some instances, all or part of the computing subsystem 110 can beimplemented as a component of, or can be integrated with one or morecomponents of, the surface equipment 112, the logging tool 102 or both.In some cases, the computing subsystem 110 can be implemented as one ormore computing structures separate from the surface equipment 112 andthe logging tool 102.

In some implementations, the computing subsystem 110 is embedded in thelogging tool 102, and the computing subsystem 110 and the logging tool102 can operate concurrently while disposed in the wellbore 104. Forexample, although the computing subsystem 110 is shown above the surface106 in the example shown in FIG. 1A, all or part of the computingsubsystem 110 may reside below the surface 106, for example, at or nearthe location of the logging tool 102.

The well system 100 a can include communication or telemetry equipmentthat allows communication among the computing subsystem 110, the loggingtool 102, and other components of the NMR logging system 108. Forexample, the components of the NMR logging system 108 can each includeone or more transceivers or similar apparatus for wired or wireless datacommunication among the various components. For example, the NMR loggingsystem 108 can include systems and apparatus for optical telemetry,wireline telemetry, wired pipe telemetry, mud pulse telemetry, acoustictelemetry, electromagnetic telemetry, or a combination of these andother types of telemetry. In some cases, the logging tool 102 receivescommands, status signals, or other types of information from thecomputing subsystem 110 or another source. In some cases, the computingsubsystem 110 receives logging data, status signals, or other types ofinformation from the logging tool 102 or another source.

NMR logging operations can be performed in connection with various typesof downhole operations at various stages in the lifetime of a wellsystem. Structural attributes and components of the surface equipment112 and logging tool 102 can be adapted for various types of NMR loggingoperations. For example, NMR logging may be performed during drillingoperations, during wireline logging operations, or in other contexts. Assuch, the surface equipment 112 and the logging tool 102 may include, ormay operate in connection with drilling equipment, wireline loggingequipment, or other equipment for other types of operations.

In some implementations, the logging tool 102 includes a magnet assemblythat includes a central magnet and two end piece magnets. Examples areshown in FIGS. 2A, 2B, and 3B. The end piece magnets can be spaced apartfrom the axial ends of the central magnet. The end pieces together withthe central magnets can define four magnetic poles, which may bearranged to enhance the static magnetic field in a volume of interest.In some cases, the central magnet defines a first magnetic fieldorientation, and the end piece magnets define a second magnetic fieldorientation that is orthogonal to the first magnetic field orientation.The logging tool 102 can also include multiple orthogonaltransversal-dipole antennas. The orthogonal transversal-dipole antennascan produce circular polarized excitation in a subterranean volume andacquire a response from the volume by quadrature coil detection.

In some implementations, the logging tool 102 includes a magnet assemblythat produces a magnetic field in multiple distinct sub-volumes in thesubterranean region 120. An example is shown in FIG. 2B. A firstsub-volume can be an elongate cylindrical-shell region that extends inthe longitudinal direction (parallel to the wellbore axis), and themagnetic field in the first sub-volume can be substantially uniformlyoriented along the longitudinal direction. Second and third sub-volumescan be spaced apart from the axial ends of the first sub-volume, and thestatic magnetic field in the second and third sub-volumes can have aradial orientation (perpendicular to the longitudinal direction). Thesecond and third sub-volumes can be located at a different distance fromthe center of the tool string than the first volume. In some instances,the locations of the second and third sub-volumes allow the logging toolto collect information for mud filtrate invasion profiling. The loggingtool 102 can also include multiple antenna assemblies at respectivelocations along the longitudinal axis. Each of the antenna assembliescan detect an NMR response from a respective one of the distinctsub-volumes.

In some implementations, the logging tool 102 includes a magnet assemblyand a transversal-dipole and monopole antenna assembly. An example isshown in FIG. 3B. The transversal-dipole and monopole antenna assemblycan obtain a unidirectional azimuthally-selective NMR response from asubterranean volume about the magnet assembly. The transversal-dipoleand monopole antenna assembly can include orthogonal transversal-dipoleantennas and a monopole antenna.

In some examples, NMR logging operations are performed during wirelinelogging operations. FIG. 1B is a diagram of an example well system 100 bthat includes an NMR tool in a wireline logging environment, accordingto various embodiments. In some example wireline logging operations, thesurface equipment 112 includes a platform above the surface 106 equippedwith a derrick 132 that supports a wireline cable 134 that extends intothe wellbore 104. Wireline logging operations can be performed, forexample, after a drill string is removed from the wellbore 104, to allowthe wireline logging tool 102 to be lowered by wireline or logging cableinto the wellbore 104.

In some examples, NMR logging operations are performed during drillingoperations. FIG. 1C is a diagram of an example well system 100 c thatincludes an NMR tool in a logging while drilling (LWD) environment,according to various embodiments. Drilling is commonly carried out usinga string of drill pipes connected together to form a drill string 140that is lowered through a rotary table into the wellbore 104. In somecases, a drilling rig 142 at the surface 106 supports the drill string140, as the drill string 140 is operated to drill a wellbore penetratingthe subterranean region 120. The drill string 140 may include, forexample, a Kelly, drill pipe, a bottom hole assembly, and othercomponents. The bottom hole assembly on the drill string may includedrill collars, drill bits, the logging tool 102, and other components.The logging tools may include measuring while drilling (MWD) tools, LWDtools, and others.

In some implementations, the logging tool 102 includes an NMR tool forobtaining NMR measurements from the subterranean region 120. As shown,for example, in FIG. 1B, the logging tool 102 can be suspended in thewellbore 104 by a coiled tubing, wireline cable, or another structurethat connects the tool to a surface control unit or other components ofthe surface equipment 112. In some example implementations, the loggingtool 102 is lowered to the bottom of a region of interest andsubsequently pulled upward (e.g., at a substantially constant speed)through the region of interest. As shown, for example, in FIG. 1C, thelogging tool 102 can be deployed in the wellbore 104 on jointed drillpipe, hard wired drill pipe, or other deployment hardware. In someexample implementations, the logging tool 102 collects data duringdrilling operations as it moves downward through the region of interest.In some example implementations, the logging tool 102 collects datawhile the drill string 140 is moving, for example, while it is beingtripped in or tripped out of the wellbore 104.

In some implementations, the logging tool 102 collects data at discretelogging points in the wellbore 104. For example, the logging tool 102can move upward or downward incrementally to each logging point at aseries of depths in the wellbore 104. At each logging point, instrumentsin the logging tool 102 perform measurements on the subterranean region120. The measurement data can be communicated to the computing subsystem110 for storage, processing, and analysis. Such data may be gathered andanalyzed during drilling operations (e.g., during logging while drilling(LWD) operations), during wireline logging operations, or during othertypes of activities.

The computing subsystem 110 can receive and analyze the measurement datafrom the logging tool 102 to detect properties of various subsurfacelayers 122. For example, the computing subsystem 110 can identify thedensity, viscosity, porosity, material content, or other properties ofthe subsurface layers 122 based on the NMR measurements acquired by thelogging tool 102 in the wellbore 104.

In some implementations, the logging tool 102 obtains NMR signals bypolarizing nuclear spins in the subterranean region 120 and pulsing thenuclei with a radio frequency (RF) magnetic field. Various pulsesequences (i.e., series of radio frequency pulses, delays, and otheroperations) can be used to obtain NMR signals, including the CarrPurcell Meiboom Gill (CPMG) sequence (in which the spins are firsttipped using a tipping pulse followed by a series of refocusing pulses),the Optimized Refocusing Pulse Sequence (ORPS) in which the refocusingpulses are less than 180°, a saturation recovery pulse sequence, andother pulse sequences.

The acquired spin-echo signals (or other NMR data) may be processed(e.g., inverted, transformed, etc.) to a relaxation-time distribution(e.g., a distribution of transverse relaxation times T₂ or adistribution of longitudinal relaxation times T₁), or both. A diffusionaxis D is also possible. The relaxation-time distribution can be used todetermine various physical properties of the formation by solving one ormore inverse problems. In some cases, relaxation-time distributions areacquired for multiple logging points and used to train a model of thesubterranean region. In some cases, relaxation-time distributions areacquired for multiple logging points and used to predict properties ofthe subterranean region.

FIG. 2A is a diagram of an example downhole tool 200A for obtaining NMRdata from a subterranean region, according to various embodiments. Theexample NMR tool 200A includes a magnet assembly that generates a staticmagnetic field to produce polarization, and an antenna assembly that (a)generates a radio frequency (RF) magnetic field to generate excitation,and (b) acquires NMR signals. In the example shown in FIG. 2A, themagnet assembly that includes the end piece magnets 11A, 11B and acentral magnet 12 generates the static magnetic field in the volume ofinvestigation 17. In the volume of investigation 17, the direction ofthe static magnetic field (shown as the solid black arrow 18) isparallel to the longitudinal axis of the wellbore. In some examples, amagnet configuration with double pole strength can be used to increasethe strength of the magnetic field (e.g., up to 100-150 Gauss or higherin some instances).

In the example shown in FIG. 2A, the antenna assembly 13 includes twomutually orthogonal transversal-dipole antennas 15, 16. In someinstances, the NMR tool 200A can be implemented with a singletransversal-dipole antenna. For example, one of the transversal-dipoleantennas 15, 16 may be omitted from the antenna assembly 13. The exampletransversal-dipole antennas 15, 16 shown in FIG. 2A are placed on anouter surface of a soft magnetic core 14, which is used for RF magneticflux concentration. The static magnetic field can be axially symmetric(or substantially axially symmetric), and therefore may not requirebroader band excitation associated with additional energy loss. Thevolume of investigation can be made axially long enough and thick enough(e.g., 20 cm long, and 0.5 cm thick in some environments) to provideimmunity or otherwise decrease sensitivity to axial motion, lateralmotion, or both. A longer sensitivity region can enable measurementwhile tripping the drill string. The sensitivity region can be shaped byshaping the magnets 11A, 11B, 12 and the soft magnetic material of thecore 14.

In some implementations, the antenna assembly 13 additionally oralternatively includes an integrated coil set that performs theoperations of the two transversal-dipole antennas 15, 16. For example,the integrated coil may be used (e.g., instead of the twotransversal-dipole antennas 15, 16) to produce circular polarization andperform quadrature coil detection. Examples of integrated coil sets thatcan be adapted to perform such operations include multi-coil or complexsingle-coil arrangements, such as, for example, birdcage coils commonlyused for high-field magnetic resonance imaging (MRI).

Compared to some example axially-symmetrical designs, the use of thelongitudinal-dipole magnet and the transversal-dipole antenna assemblyalso has an advantage of less eddy current losses in the formation anddrilling fluid (i.e., “mud”) in the wellbore due to a longer eddycurrent path than for some longitudinal-dipole antenna(s).

In some aspects, NMR measurements over multiple sub-volumes can increasethe data density and therefore SNR per unit time. Multiple volumemeasurements in a static magnetic field having a radial gradient can beachieved, for example, by acquiring NMR data on a second frequency whilewaiting for nuclear magnetization to recover (e.g., after a CPMG pulsetrain) on a first frequency. A number of different frequencies can beused to run a multi-frequency NMR acquisition involving a number ofexcitation volumes with a different depth of investigation (DOI). Inaddition to higher SNR, the multi-frequency measurements can also enableprofiling the fluid invasion in the wellbore, enabling a betterassessment of permeability of earth formations. Another way to conductmulti-volume measurements is to use different regions of the magnetassembly to acquire an NMR signal. NMR measurements of these differentregions can be run at the same time (e.g., simultaneously) or atdifferent times.

FIG. 2B is a diagram of another example downhole tool 200B for obtainingNMR data from a subterranean region, according to various embodiments.The example NMR tool 200B also includes a magnet assembly that generatesa static magnetic field to produce polarization, and an antenna assemblythat (a) generates a radio frequency (RF) magnetic field to generateexcitation, and (b) acquires NMR signals. In the example shown in FIG.2B, the magnet assembly produces a magnetic field having a dominantaxial component in the volume of investigation 21. The directions of theRF magnetic field (produced by two transversal dipole antennas as inFIG. 2A) and the static magnetic field in this region are shown at 22.In the example shown in FIG. 2B, two distinct volumes of investigation24A, 24B are created near the magnet poles (beyond the axial ends of thecentral magnet) where the static magnetic field has a predominantlyradial component. The example NMR antennas shown at 23A and 23B cangenerate RF magnetic fields in the volumes of investigation 24A and 24Bnear the longitudinal-dipole antennas. The longitudinal direction of theRF magnetic fields in the volumes of investigation 24A and 24B, and theradial direction of the static magnetic field in the volumes ofinvestigation 24A and 24B, are shown at 25A and 25B.

In some aspects, a combination of transversal-dipole and monopoleantennas can be used to enable unidirectional azimuthally-selectivemeasurements, without substantially reducing SNR in some cases. In someexamples, the NMR excitation can be substantially axially symmetrical(e.g., using either the transversal-dipole antenna or the monopoleantenna) while a combination of axially-symmetrical sensitivitytransversal-dipole antenna and the axially-symmetrical sensitivitymonopole antenna responses can enable azimuthally-resolved measurements.

FIGS. 3A and 3B illustrate aspects of an example azimuthally-selectiveNMR tool. FIG. 3A is a plot 300A showing azimuthal selectivity for anexample downhole tool 300B, according to various embodiments. Theexample NMR tool 300B includes a magnet assembly that generates a staticmagnetic field to produce polarization, and an antenna assembly that (a)generates a radio frequency (RF) magnetic field to generate excitation,and (b) acquires NMR signals. The antenna assembly 31 shown in FIG. 3Bincludes a substantially monopole antenna and two orthogonaltransversal-dipole antennas 35 and 36. The example monopole antennaincludes two coils 37A and 37B connected in reverse polarity in order togenerate a substantially radial RF magnetic field in the volume ofinvestigation 34. Due to reciprocity, the same coil arrangement can havea radial sensitivity direction. The example RF magnetic fields BRFpresented at 32 and 33 can reflect the total sensitivity direction whenthe monopole antenna response is combined with one of thetransversal-dipole antenna responses.

FIG. 3B is a diagram of another example downhole tool for obtaining NMRdata from a subterranean region, according to various embodiments. Theexample monopole antenna shown in FIG. 3B includes an arrangement ofcoils that generate locally a substantially radially-directed magneticfield, i.e., the field that would be produced by a single “magneticcharge” or magnetic pole. Here, the term “monopole” is used todistinguish this type of magnetic field from a dipole magnetic field(transversal or longitudinal). In some cases, the monopole antennaassembly generates quasi-stationary (relatively low frequency) magneticfields. In the example shown, the coils 37A and 37B, which are connectedin reverse polarity, are two parts of one monopole antenna assembly.Each coil by itself can be implemented as a standard longitudinalantenna. A monopole antenna can be implemented in another manner.

The polar plot in FIG. 3A shows an example of the antenna sensitivity,demonstrating unidirectional azimuthal selectivity. A combination of theresponses of each of the orthogonal transversal-dipole antennas with theresponse of the monopole antenna can give any of four possibledirections covering all quadrants of the transversal plane. Rotation ofthe drill string while drilling may cause an amplitude modulation of theazimuthally selective response and therefore an amplitude modulation ofthe NMR relaxation signal (e.g., a CPMG echo train). The amplitudemodulation parameters can indicate the azimuthal variations of the NMRproperties (e.g., the NMR porosity variations).

The coils 37A and 37B of the example monopole antenna shown in FIG. 3Bcan be used in combination with transversal-dipole antennas 35 and 36,for example, to achieve azimuthal selectivity. Either of the coils 37Aand 37B can also be used as a separate antenna (in addition to orwithout the transversal-dipole antennas 35, 36), for example, to gainSNR. In some cases, an NMR tool is implemented with a monopole antennaand a longitudinal magnet, without other antennas. For example, thetransversal-dipole antennas 35 and 36 may be omitted from the antennaassembly 31 in some cases.

FIG. 4A is a flowchart showing an example technique 400 for obtainingNMR data from a subterranean region, according to various embodiments.FIG. 4B is a flowchart showing another example technique 420 forobtaining NMR data from a subterranean region, according to variousembodiments. Each of the processes 400 and 420 can be performedindependent of the other, or the processes 400 and 420 can be performedconcurrently or in concert. For example, the processes 400 and 420 maybe performed in series or in parallel, or one of the processes may beperformed without performing the other.

The processes 400 and 420 can be performed by downhole NMR tools such asthe example NMR tools 200A, 200B, or 300B shown in FIGS. 2A, 2B and 3B,or by another type of NMR tool. The processes 400 and 420 can beperformed by a downhole NMR tool while the tool is disposed within awellbore during well system operations. For example, the downhole NMRtool can be suspended in the wellbore for wireline logging (e.g., asshown in FIG. 1B), or the downhole NMR tool can be coupled to a drillstring for NMR LWD (e.g., as shown in FIG. 1C).

Each of the processes 400 and 420 can include the operations shown inFIGS. 4A and 4B (respectively) or either of the processes can includeadditional or different operations. The operations can be performed inthe order shown in the respective figures or in another order. In somecases, one or more of the operations can be performed in series orparallel, during overlapping or non-overlapping time periods. In somecases, one or more of the operations can be iterated or repeated, forexample, for a specified number of iterations, for a specified timeduration, or until a terminating condition is reached.

At 402 in the example process 400 shown in FIG. 4A, the NMR tool ispositioned in a wellbore. In some cases, the NMR tool includes a magnetassembly to produce a magnetic field in a volume in the subterraneanregion about the wellbore. The volume can include, for example, all orpart of any of the volumes of investigation 17, 21, 24A, 24B, 34 shownin FIG. 2A, 2B or 3B, or another volume of interest. Generally, the NMRtool includes a magnet assembly to polarize nuclear spins in the volumeof interest, and an antenna assembly to excite the nuclear spins and toacquire an NMR signal based on the excitation.

At 404, polarization is generated in a volume about the wellbore. Thepolarization is generated by a static magnetic field, which is producedby the magnet assembly of the NMR tool in the wellbore. The polarizationrefers to the magnetic polarization of the nuclear spins in the volume.In other words, a portion of the nuclear spins becomes aligned with thestatic magnetic field, and the volume develops a bulk magnetic moment.In some cases, the static magnetic field is configured (e.g., by theshape and position of the magnet assembly) to produce longitudinalpolarization (e.g., parallel to the long axis of the wellbore) orpolarization having another orientation.

In some examples, the magnet assembly includes a central magnet (e.g.,the central magnet 12 shown in FIGS. 2A, 2B, 3B, or another type ofcentral magnet) and two end piece magnets (e.g., the end piece magnets11A, 11B shown in FIGS. 2A, 2B, 3B, or another type of end piecemagnet). In some cases, the magnets in the magnet assembly are permanentmagnets. As shown, for example, in FIG. 2A, the central magnet can be anelongated permanent magnet having a first axial end and a second,opposite axial end, with the first end piece magnet spaced apart fromthe first axial end of the central magnet, and with the second end piecemagnet spaced apart from the second axial end of the central magnet. Insome cases, the two end piece magnets have a common magnetic fieldorientation, and the central magnet has the opposite magnetic fieldorientation (e.g., such that both end piece magnets have a magneticfield orientation that is orthogonal to the magnetic field orientationof the central magnet).

At 406, circular-polarized excitation is generated in the volume aboutthe wellbore. The circular-polarized excitation is produced in thevolume by an antenna assembly. For example, the antenna assembly can beenergized by an oscillating current in the radio frequency range whichproduces a radio-frequency (RF) magnetic field in the volume about thewellbore. Circular polarization is an attribute of using two antennaewhere the resulting alternating field rotates. In circular polarizationthe RF field maintains its magnitude, but the orientation rotates arounda particular vector. This is in contrast to a single antenna which has afield in one direction (e.g., linear) that grows and shrinks along aline. The RF magnetic field generated by the antenna assemblymanipulates the nuclear spins to produce an excited spin state.

In some examples, the antenna assembly includes orthogonaltransversal-dipole antennas. The antenna assembly 13 shown in FIGS. 2Aand 2B and the antenna assembly 31 shown in FIG. 3B are examples ofantenna assemblies that include two orthogonal transversal-dipoleantennas. Each antenna 15, 16 in the example antenna assembly 13 canindependently produce a transversal-dipole magnetic field, for example,by conducting radio-frequency current. In the examples shown, eachtransversal-dipole magnetic field has a transverse orientation withrespect to the longitudinal axis of the NMR tool. In other words, thetransversal-dipole magnetic field is oriented orthogonal to the longaxis of the wellbore.

In the example shown, the transversal-dipole magnetic field produced bythe antenna 15 is orthogonal to the transversal-dipole magnetic fieldproduced by the other antenna 16. For example, in a Cartesian coordinatesystem of three mutually-orthogonal directions, the longitudinal axis ofthe NMR tool can be considered the “z” direction, and thetransversal-dipole magnetic fields (produced by the antennas 15, 16) areoriented along the “x” and “y” directions, respectively.

In some implementations, multiple excitations are produced by the NMRtool. For example, in some cases, the circular-polarized excitation isproduced in a first sub-volume (e.g., the volume of investigation 21 inFIG. 2B) by the orthogonal transversal-dipole antennas, and excitationhaving another orientation is produced in second and third sub-volumes(e.g., the volumes of investigation 24A, 24B in FIG. 2B) that are spacedapart from the axial ends of the first sub-volume. The excitation in thesecond and third sub-volumes can be produced, for example, by alongitudinal-dipole RF field generated by other antenna assemblies(e.g., by antennas 23A and 23B in FIG. 2B). The distinct sub-volumes maybe useful for different purposes. For example, the first sub-volume canbe elongate (parallel to the long axis of the wellbore), to acquire NMRdata from the first sub-volume while the NMR tool moves along thewellbore (e.g., while tripping a drill string). In some cases, the othersub-volumes can be positioned to acquire NMR data for mud filtrateinvasion profiling or other applications.

At 408, an NMR signal is acquired by quadrature coil detection. The NMRsignal is based on the excitation generated at 406. The NMR signal canbe, for example, an echo train, a free induction decay (FID), or anothertype of NMR signal. In some cases, the acquired NMR data includes T₁relaxation data, T₂ relaxation data, or other data. The NMR signal canbe acquired by the antenna assembly that produced the excitation or byanother antenna assembly. In some cases, an NMR signal can be acquiredin multiple sub-volumes.

Quadrature coil detection can be performed by the orthogonaltransversal-dipole antennas. Quadrature coil detection can be performedby using two orthogonal coils, each picking up the signal induced bycircular polarized nuclear magnetization (the signal in the coils have90 degree phase difference). Even if during transmission only one coilis used (e.g., producing linear polarized RF magnetic field), thenuclear magnetization can still be circular polarized—by disposing ofhalf of the linear field. Quadrature coil transmission (two orthogonalcoils driven by RF currents having 90 degree phase difference) canenable circular polarized excitation, which can help to reduce powerconsumption compared to a linear polarized excitation in some cases.Quadrature coil detection can be used, for example, to increasesignal-to-noise ratio (SNR) when exciting only one coil (not usingcircular polarized excitation to simplify hardware), or circularpolarization can be used to save power while detecting signals with onecoil. In some cases, both circular polarization and quadrature coildetection can be used to save power and increase SNR. In some cases, theuse of circular polarization or quadrature coil detection (or both) isefficient when the mutually orthogonal antennas are substantiallyidentical. This is possible in the example magnet/antenna configurationthat has a longitudinal dipole magnet and two transversal antennae.Other configurations that have one of the two antennae less efficientthan the other, although allowing for mutually orthogonal antennae, maynot provide the same advantages in some cases.

At 410, the NMR data are processed. The NMR data can be processed toidentify physical properties of the subterranean region or to extractother types of information. For example, the NMR data may be processedto identify density, viscosity, porosity, material content, or otherproperties of the subterranean region about the wellbore.

At 422 in the example process 420 shown in FIG. 4B, the NMR tool ispositioned in a wellbore, and at 424 polarization is generated in avolume about the wellbore. Operations 422 and 424 in FIG. 4B are similarto operations 402 and 404 shown in FIG. 4A. For example, the NMR toolincludes a magnet assembly to polarize nuclear spins in the volume ofinterest, and an antenna assembly to excite the nuclear spins and toacquire an NMR signal based on the excitation. The polarization can beproduced at 424 in the manner described with respect to operation 404 ofFIG. 4A and by the same type of magnet assembly; or polarization can beproduced at 424 in another manner or by another type of magnet assembly.

At 426, excitation is generated in a volume about the wellbore. Theexcitation is produced in the volume by an antenna assembly. Forexample, the antenna assembly can be energized by a radio-frequencycurrent, which produces a radio-frequency (RF) magnetic field in thevolume about the wellbore. The RF magnetic field generated by theantenna assembly manipulates the nuclear spins to produce an excitedspin state. In some instances, the spin state has a higher excitation ina selected azimuthal direction, such that the level of spin excitationvaries along a circular (or circumferential) direction about thewellbore, for example, due to an azimuthally-selective RF magneticfield.

In some examples, the antenna assembly includes a transversal-dipole andmonopole antenna assembly. The antenna assembly 31 shown in FIG. 3B isan example of an antenna assembly that includes a transversal-dipole andmonopole antenna assembly. In the example shown in FIG. 3B, thetransversal-dipole and monopole antenna assembly includes two orthogonaltransversal-dipole antennas 35 and 36 in a central region, and amonopole antenna that includes a first coil 37A at a first axial end ofthe transversal-dipole antennas 35 and 36 and a second coil 37B at asecond, opposite axial end of the transversal-dipole antennas 35 and 36;the coils 37A and 37B of the monopole antenna are arranged with oppositepolarity.

At 428, an azimuthally-selective NMR signal is acquired. The NMR signalis based on the excitation generated at 426. The NMR signal can be, forexample, an echo train, a free induction decay (FID), or another type ofNMR signal. In some cases, the acquired NMR data includes T₁ relaxationdata, T₂ relaxation data, or other data. The NMR signal can be acquiredby the antenna assembly that produced the excitation or by anotherantenna assembly. In some cases, the NMR signal is acquired by anantenna assembly having azimuthally-selective sensitivity, such as, atransversal-dipole and monopole antenna assembly.

In some implementations, the azimuthally-selective NMR signal isacquired as a combination of multiple NMR signal acquisitions. Thesignal acquisitions can include, for example, acquisitions by one ormore transversal-dipole antennas and one or more monopole antennas. Thesignals can be combined to enable azimuthally-resolved measurements ofthe volume about the wellbore. For example, in some cases, a propercombination of the responses of each of the orthogonaltransversal-dipole antennas with the response of the monopole antennacan give any of four possible directions covering all quadrants of thetransversal plane.

At 430, the NMR data are processed. The NMR data can be processed toidentify physical properties of the subterranean region or to extractother types of information. For example, the NMR data may be processedto identify density, viscosity, porosity, material content, or otherproperties of the subterranean region about the wellbore. In some cases,the NMR data are processed to identify azimuthal variations in thesubterranean region about the wellbore. For example, rotating the NMRtool may cause an amplitude modulation of the azimuthally-selectiveresponse. The amplitude modulation parameters can indicate the azimuthalvariations of the properties affecting the NMR signal (e.g., porosity,density, viscosity, material content, etc.). Further embodiments may berealized.

For example, conventional methods of using NMR-based tools have beendiscussed in the literature, but the practical implementation of theseconcepts was not fully developed. More specifically, the followingdiscussion will present various embodiments that provide a practical wayto realize multiple sensitive zones along an NMR tool which has centralcircular polarized transverse antennas and one or more longitudinalmagnets.

As a mechanism for comparison, FIG. 5 is a schematic view of an NMR toolconfiguration, according to various embodiments. FIG. 6 is a wireframemodel view of the NMR tool configuration shown in FIG. 5. This schematicand wireframe view of tool construction represent an alternative versionof the perspective view for the construction shown for NMR tools 200A,200B, or 300B in FIGS. 2A, 2B and 3B. In these embodiments,tangentially-radial, symmetric magnetic fields 502 are created by athree-section magnet assembly 504 comprising a central magnet 510, afirst end piece magnet 520, and a second end piece magnet 530.

A section of the cylindrical central magnet 510 has been broken out inthe drawing as ring magnet 510′ and pseudo-cylinder magnet 510″ to showthat although the schematic view provides two-dimensional sections, themagnets (and other component elements of the assembly 404, including thefields 502) are three-dimensional in nature. Thus, magnet configurationshenceforth will be represented as a slice cut along the longitudinalaxis 560 of a downhole tool. These slices represent a full cylinder asdemonstrated in FIGS. 5 and 6, and not demonstrated in the subsequentdiagrams as it is understood that all drawing of the magnet assembliesand tools hereafter are represented by a two-dimensional slice.

The NMR sensitive volume in this case is excited using two antennae (notshown, but illustrated as elements 15 and 16 in FIG. 2A, for example) tocreate the fields 502 utilizing the principles of circular polarization.To shape the fields 502, the magnets 510, 520, 530 and magneticallypermeable material 550 can be shaped. The fields 502 can also be shapedby adding shim magnets to the basic configuration shown in FIG. 5, aswill be described in subsequent figures.

For example, FIG. 7 illustrates side, cut-away views 710, 720, 730 of aportion of a radially-symmetric magnetic field profile, according tovarious embodiments. The secondary zones can be shaped to form a singlesaddle point, a double saddle point, a multi-saddle point, or beyond asaddle point. For example, in the views 710, 720, 730 the fields are allradially symmetric. Each view shows a slice radially outward and alongthe tool's longitudinal axis, where the fields have been shaped to formwhat is known to those of ordinary skill in the art as a pillow type(e.g., view 710), a butterfly type (e.g., view 720), and apast-saddle-point” type (e.g., view 730).

Secondary zones such as these may be formed using two long sets ofmagnets where the same pole-sides face each other, perhaps using aJasper-Jackson design that is known to those of ordinary skill in theart. For example, two long sets of ring magnets having a length of 0.1 mcould be spaced about 0.1 m apart, with the Southern poles facing eachother. Two long magnets in this position will be called “pole” magnetshenceforth. A magnetically permeable material may be placed in thecenter of this configuration (e.g., see material 550 in FIG. 5). Thismaterial has many uses, one of which is shaping a magnetic field.However, using only the three-part configuration shown in FIG. 5,adjusting the magnet lengths, separating the poles, and magneticallypermeable material configuration is sometimes not enough to provide theDOI or gradient desired, with respect to the effective NMR sensitivevolume. Thus, although shaping the fields 502 is difficult, it cansometimes be accomplished by adding magnetically permeable material overthe outside of each of the magnets. This action may be sufficient tofocus the antenna field outward and, at the same time, reduce themagnitude of the B1 field towards the magnets. However, this is notalways possible.

For example, FIG. 8 illustrates side, cut-away views 810, 820 of aportion of a modeled radially-symmetric magnetic field profile,according to various embodiments. In this case, the views 810, 820correspond to shaping fields using the configuration of FIG. 5. Thesingle active NMR zone 830 is long and deep. When magnet lengths andpole separation are adjusted, and magnetically permeable material areapplied to the configuration shown in FIG. 5, three zones 840, 850, 860exist, at the same frequency. Unfortunately, the saddle point zone 850falls into the borehole. While this profile may be useful may be usefulfor measurement in some cases, it is not generally desired.

FIG. 9 illustrates side, cut-away views 910, 920, 930 of a portion of amodeled radially-symmetric magnetic field profile, as pole magnets moveaway from the center, according to various embodiments. Here it can beseen that as the pole magnets are moved further away from the center(i.e., the central magnet in a three-magnet configuration, such as thatshown in FIG. 5), the pillow zone 940 moves outward past the boreholewall, and the sensitive volume (spin central or antenna resonant)frequency drops.

FIG. 10 illustrates side, cut-away views 1010, 1020 of a portion of amodeled radially-symmetric magnetic field profile, as pole magnets moveaway from the center, at an operating frequency of 295 kHz, according tovarious embodiments. FIG. 11 illustrates side, cut-away views 1110, 1120of a portion of a modeled radially-symmetric magnetic field profile, aspole magnets move away from the center, at an operating frequency of 716kHz, according to various embodiments. These views correspond toincreasing the central magnet outside diameter and separating the polemagnets from the central magnet, to create a secondary zone that isradially-directed and past the borehole wall. The total magnet span inthis case approach three meters. However, the resulting sensitive volumefrequency for many situations would be less than is desirable.

Thus, a three-magnet configuration (shown in FIG. 5) is unlikely to givedesirable depths of investigation (see FIG. 8), desirable NMRoperational frequency (see FIG. 9), a desirable tool length, and a deepDOI (see FIG. 10) at the same time for the top 1030, bottom 1040, andcenter 1050 NMR active zones. In practice, with only three magnets, onlya single saddle point (i.e., zones 1050, 1150) will arise naturallywithin the gap between the middle and top/bottom magnets. However, theaddition of more magnets and permeable material can help control thetype of fields created, along with the total length of the tool, toprovide a more desirable result. The following figures demonstrate somenon-limiting examples.

FIGS. 12-13 are schematic views of NMR tool configurations 1200, 1300,according to various embodiments. Here the configurations 1200, 1300 mayinclude any and all of the elements shown in configurations 200A, 200Band 300B, shown in FIGS. 2A, 2B, and 3C, respectively. In addition,magnetically permeable material 1210, such as ferrite, or Ferrotron 559Hsoft magnetic composite material available from Fluxtrol, Inc. of AuburnHills, Mich., and/or one or more shim magnets 1220 are disposed betweenthe central magnet 510 and the first and second end piece magnets 520,530, respectively. By adding small inner magnets (e.g., magnets 1220)between the central magnet 510 and the end piece magnets 520, 530, aradially (i.e., the dominant field direction) sensitive zone with adeeper and more desirable DOI can be created.

Indeed, many variations are possible. For example, in the toolconfiguration 1200, the fields 1230 and DOI are symmetrical, due to thesymmetrical location and size of the magnetically permeable material1210 and shim magnets 1220. The ends of the shim magnets 1220 may opposeor align with the end piece magnets 520, 530, and the central magnet510, depending on the desired field strength and shape. The magnets 510,520, 530, 1220 may comprise rings (similar to or identical to ringmagnet 510′ in FIG. 5), rectangular sections (e.g., similar to oridentical to the pseudo-cylinder magnet 510″ shown in FIG. 5), or somearbitrary shape (e.g., triangular, square, or ellipse).

In some embodiments, the magnets 510, 520, 530, 1220 have a remnantfield orientation along the longitudinal axis 560 of the tool. In someembodiments, the remnant field orientation is radially symmetric, and inothers, tilted. For example, a pseudo-cylinder (e.g., the cylinder 510″shown in FIG. 5) can be formed via the circular arrangement of severalrectangular magnets, of the same or different sizes, around theazimuthal periphery of the tool.

In many embodiments, symmetry is not necessary, nor desirable. Thus,while the configuration 1200 is entirely symmetric, the configuration1300 is asymmetric. The asymmetry in configuration 1300 stems from theuse of different sizes of shim magnets, and different amounts ofmagnetically permeable material. Thus, the shim magnets 1320 are smallerand the amount of magnetically permeable material 1310 adjacent to themis less in the upper portion 1360 of the configuration 1300, whereas theshim magnets 1380 are larger and the amount of magnetically permeablematerial 1370 adjacent to them is greater in the lower portion 1390 ofthe configuration 1300. The result is that the fields 1330 forconfiguration 1300 are different, with a different DOI. Whereas thefields 1230 are the same, with the same DOI. In either configuration1200, 1300, one or more antennae (e.g., antennae 15 and 16 shown in FIG.2A, or antennae 35, 26, 37A, 37B in FIG. 3B) can be disposed over themagnetically permeable material 550. In some embodiments, copper isdisposed between one or more antennae (e.g., antennae 15 and 16 shown inFIG. 2A, or antennae 35, 26, 37A, 37B in FIG. 3B), and the magneticallypermeable material 550. These antennae and the copper are not shown inFIGS. 12 and 13, so as not to obscure the appearance of other elements;to view the application of these features, the reader is referred toFIG. 15.

FIG. 14 illustrates a field profile 1400 of the pillow zone in themagnetic field created by the addition of magnetically permeablematerial to the tool configuration, according to various embodiments.Here the pillow zone is created only by the addition of magneticallypermeable material between the central magnet and one of the end piecemagnets. In addition to magnets, NMR tools constructed according tovarious embodiment may be operated with a magnetically permeablematerial between the magnets and one or more antennae (see FIG. 15).This construction helps reduce antenna field penetration of the magnet,which might cause ringing. This construction also helps to focus the B1magnetic field outward into the formation, to increase the availableSNR. This configuration is essentially the same as configuration 1200 ofFIG. 12, with the magnetically permeable material 1210 present, andwithout any shim magnets 1220. The resulting radial pillow zone occursat a frequency of 205 kHz, with a total tool length of about 3 m, withthe pole magnets (end piece magnets spread further apart than is shownin FIG. 12), to obtain a larger DOI.

FIGS. 15-16 are schematic views of NMR tool configurations 1500, 1600,according to various embodiments. FIG. 17 illustrates symmetric fieldprofiles 1700, 1710 of the magnetic fields that correspond to the NMRtool configurations of FIGS. 15-16, according to various embodiments.

To this point, the advantage of using additional shim magnets 1560,disposed at the outer ends of the end piece magnets 520, 530 has notbeen discussed. When this embodiment is implemented, as shown in FIGS.15, 16, 18, and 20, instead of creating second and third sensitive zones(shown as components of the fields 502 in FIG. 5) between the two endpiece magnets of a three-magnet configuration (shown as magnets 510,520, 530 in FIG. 5), the second and third zones 1502, 1504 are createdoutside of the three-magnet configuration 510, 520, 530, while thecentral zone 1506 remains located between the two end piece magnets 520,530. In another embodiment, shim magnets 1560 are disposed adjacent toonly one of the end piece magnets (e.g., magnet 520). This optionprovides only two sensitive zones 1502, 1506.

In FIG. 15, the additional shim magnets 1560 have been added in asymmetric fashion, with their polar magnetization in opposition to thatof the proximal end piece magnets 520, 530. Disposed between the shimmagnets 1560 and the end piece magnets 520, 530 is a magneticallypermeable material 1550. One or more antennae 37A, 37B may be disposedover the magnetically permeable material 1550. As was the case in priorembodiments, antennae 35, 36 may also be disposed over the magneticallypermeable material 550. In each case where an antenna is disposed overmagnetically permeable material, copper 1570 may be placed in-betweenthe antenna and the magnetically permeable material. As can be seen inFIG. 16, additional shim magnets 1660 can be added to the configurationshown in FIG. 15 to fine-tune the sensitive field distribution, as wellas the DOI. Any number of shim magnets 1560, 1660 can be used, but inmany cases, two such magnets are sufficient to achieve inspectionobjectives. The configurations shown in FIGS. 15 and 16 permit thecentral sensitive zone 1506 to remain symmetrically uncompromised.

Again, symmetric configurations, as shown in FIGS. 15-16, may be usefulin many applications. In others, an asymmetric distribution of thesensitive volumes may be desired. Thus, the configuration in FIG. 16allows at least two types of asymmetry, if desired. In the first type,shim magnets 1660 have been added radially beneath the magneticallypermeable material 1550. In the second type, the shim magnets 1670 havebeen added longitudinally between the end piece magnet 530 and themagnetically permeable material 1680. Still more shim magnets 1690 maybe added, perhaps underneath the magnetically permeable material 1680.The shim magnets 1660, 1670, 1690 can be added to provide symmetriczones 1502, 1504, or asymmetric zones (e.g., see FIG. 18), as desired.

Thus, some embodiments (e.g., symmetrical) provide the same DOI for eachof the outer sensitive zones 1502, 1504. Some embodiments (e.g.,asymmetrical) provide a different DOI for each of the outer sensitivezones 1502, 1504. With the latter, different depths of investigation(DOIs) can be provided using the same tool. For any sensitivity zone,any number of frequencies can be used. Saddle point and past saddlepoint regions might make use of a single antenna if this embodiment isused.

FIG. 18 is a schematic view of an NMR tool configuration 1800, accordingto various embodiments. FIG. 19 illustrates asymmetric field profiles1900, 1910 of the magnetic fields that correspond to the NMR toolconfiguration of FIG. 18, according to various embodiments. In thiscase, an asymmetric configuration 1800 is employed, by using shimmagnets 1660 that are larger at one end of the tool, and shim magnets1670, 1690 that are smaller at the other end of the tool, providinglarger and smaller sensitive volumes 1502, 1504, respectively. Tomaintain an axially-symmetric sensitive zone 1506 in this configuration1800, the central and/or end piece magnets 510, 520, 530 may need to beadjusted in size.

FIG. 20 is a schematic view of an NMR tool configuration 2000, accordingto various embodiments. Here, the configurations 1200 (from FIG. 12) and1800 (from FIG. 18) have been combined, with multiple shim magnets 1220disposed longitudinally between the central magnet 510 and each of theend piece magnets 520, 530. Additional shim magnets 1660 have also beenlongitudinally disposed between the end piece magnets 520, 530 and shimmagnets 1560. The end result is symmetric, but other embodiments are notto be so limited, so the multiple DOIs can be obtained using a singletool configuration. Still further pairs of shim magnets can be used toextend the longitudinal length of configuration 2000, or any otherconfiguration described herein. There is no theoretical limit to theamount of extension, and the number of sensitive zones that arecreated—but practical limitations on the length of the tool may exist.

FIG. 21 is a block diagram of apparatus 2110 and systems 2100 accordingto various embodiments. Here, it can be seen that the system 2100 mayinclude a controller 2125 to interface with a controlled device 2170,such as a geosteering unit, and/or a user display or touch screeninterface, which may be included within, or separated from, displayunits 2155. The system 2100 may further include a number of sensorsdownhole, including antennae ANT1, ANT2, . . . , ANTN as part of theapparatus 2110, which may comprise a downhole tool 102, similar to oridentical to the tool 102 shown in prior figures (e.g., FIGS. 1A, 1B,1C). The apparatus 2110 may thus comprise a downhole tool 102constructed according to any of the configurations 200A, 200B, 300B,1200, 1300, 1500, 1600, 1800, 2000, shown in FIGS. 22A, 2B, 3B, 12, 13,15, 16, 18, and 20 respectively. The apparatus 2110 may further comprisetransmitters and/or receivers (together transceivers 2104), which may belocated on the apparatus 2110, or at a surface location, remote from theapparatus 2110. When constructed in this manner, the system 2100 canreceive measurements and other data (e.g., corresponding to NMRmeasurements in a geological formation) to be processed according tovarious methods described herein.

The processing unit 2102 can be coupled to the transceivers 2104 toobtain measurements from the antennae ANT1, ANT2, . . . , ANTN, amongothers. The processing unit 2102 may be part of a surface workstation orattached to a downhole tool 102 housing, as part of the apparatus 2110.

The logging system 2100 can include a controller 2125, other electronicapparatus 2165, and a communications unit 2140. The controller 2125 andthe processing unit 2102 can be fabricated to operate the antennae ANT1,ANT2, . . . , ANTN to acquire measurement data, such as signalsrepresenting sensor measurements, perhaps resulting from NMR excitationof the surrounding formation.

Electronic apparatus 2165 (e.g., electromagnetic sensors, currentsensors) can be used in conjunction with the controller 2125 to performtasks associated with taking measurements downhole. The communicationsunit 2140 can include downhole communications in a drilling operation.Such downhole communications can include telemetry.

The system 2100 can also include a bus 2127 to provide common electricalsignal paths between the components of the system 2100. The bus 2127 caninclude an address bus, a data bus, and a control bus, eachindependently configured. The bus 2127 can also use common conductivelines for providing one or more of address, data, or control, the use ofwhich can be regulated by the controller 2125.

The bus 2127 can include instrumentality for a communication network.The bus 2127 can be configured such that the components of the system2100 are distributed. Such distribution can be arranged between downholecomponents such as the transceivers 2104 and components that can bedisposed on the surface of a well. Alternatively, several of thesecomponents can be co-located, such as on one or more collars of a drillstring, of which the tool 102 forms a part.

In various embodiments, the system 2100 includes peripheral devices thatcan include displays 2155, additional storage memory, or othercontrolled devices 2170 that may operate in conjunction with thecontroller 2125 or the processing unit 2102. The display 2155 candisplay diagnostic and measurement information for the system 2100,based on the signals generated according to embodiments described above.

In an embodiment, the controller 2125 can be fabricated to include oneor more processors. The display 2155 can be fabricated or programmed tooperate with instructions stored in the processing unit 2102 (forexample in the memory 2106) to implement a user interface to manage theoperation of the system 2100, including any one or more componentsdistributed within the system 2100. This type of user interface can beoperated in conjunction with the communications unit 2140 and the bus2127. Various components of the system 2100 can be integrated with thebottom hole assembly shown in FIG. 1C, which may in turn be used tohouse the magnets, magnetically permeable material, antennae ANT1, ANT2,. . . , ANTN, etc. as shown in the various configurations 200A, 200B,300B, 1200, 1300, 1500, 1600, 1800, 2000 described herein, such that theoperations and processing identical to or similar to the methodsdiscussed previously, and those that follow, can be conducted. Thus,various embodiments may be realized.

For example, referring now to FIGS. 1-21, it can be seen that in someembodiments, an apparatus 2110 comprises a downhole tool 102 attached toa magnet assembly that includes a central magnet 510 in-between two endpiece magnets 520, 530, and one or more shim magnets 1220 withmagnetically permeable material 1210 located in-between the centralmagnet 510 and one of the end piece magnets 520, 530 (e.g., see FIG.12). The magnetically permeable material 1210 can be disposed over amagnet 1220, at the same longitudinal location as the magnet, as shownin FIG. 12, or as shown in FIG. 15, the magnetically permeable material1550 may be longitudinally offset from the location of the shim magnet1560. In some embodiments, the magnetically permeable material 550 isdisposed over the central magnet 510, and the magnetically permeablematerial 12010 is also disposed over the shim magnets 1220, so that thelongitudinal location of the magnetically permeable material 1210 andthe respective shim magnets 1220 is approximately the same (e.g., seeFIG. 12). In some embodiments, the magnetically permeable material 550is disposed over the central magnet 510, and the magnetically permeablematerial 1550 is also longitudinally offset from the shim magnets 1560(exposing the shim magnets 1560 directly to the formation), so that thelongitudinal location of the magnetically permeable material 1550 andthe respective shim magnets 1560 are different (See FIG. 15).

Thus, in some embodiments, an apparatus 2110 comprises a magnet assemblyto produce a magnetic field 1230 in a volume in a geological formation,the magnet assembly attached to a downhole tool 102, with the magnetassembly comprising a central magnet 510 having a first axial end and asecond, opposite axial end; a first end piece magnet 520 spaced apartfrom the first axial end of the central magnet 510; and a second endpiece magnet 530 spaced apart from the second axial end of the centralmagnet 510. The apparatus 2110 further comprises at least one first shimmagnet 1220 or 1320 longitudinally disposed between the central magnet510 and the first end piece magnet 520, the first shim magnet 1220 or1320 disposed adjacent to or at least partially surrounded by a firstmagnetically permeable material 1210 or 1310, respectively, the firstshim magnet to shape a first static magnetic field sub-volume 1230, 1330provided by the central magnet 510 and the first end piece magnet 510.

Copper can be used to shield the central magnet and/or shim magnets fromthe RF antenna pulses. Thus, in some embodiments, the apparatus 2110comprises copper 1570 radially disposed between the first shim magnet1220 and the first magnetically permeable material 1210, or between thecentral magnet 510 and central magnetically permeable material 550.

Shim magnets can be formed as integral rings, or separate elements thatcombine to form a ring-like structure. Thus, in some embodiments, the atleast one first shim magnet comprises multiple unitary ring-shapedelements (e.g., in ring 510′) and/or a series of rectangular elementsshaped to form a ring (e.g., in ring 510″).

A second shim magnet can be added, to mirror the location of the firstshim magnet along the longitudinal axis of the tool. Additionalmagnetically permeable material may be disposed adjacent to or at leastpartially surrounding the second shim magnet. Thus, in some embodiments,the apparatus 2110 comprises at least one second shim magnet 1670disposed between the central magnet 510 and the second end piece magnet530, the second shim magnet 1380 disposed adjacent second magneticallypermeable material 1370 or at least partially surrounded by the secondmagnetically permeable 1370 material and copper 1570 (not shown in FIG.13, but see copper 1570 disposed between the magnets 1220 andmagnetically permeable material 1210 in FIG. 20).

Additional sets of shim magnets, and magnetically permeable material,can be added to symmetrically and outwardly extend the pattern set bythe first and second shim magnets, to whatever distance is desired.Thus, in some embodiments, the apparatus 2110 comprises at least one setof additional shim magnets 1660 adjacent to or at least partiallysurrounded by additional magnetically permeable material 1550, tooutwardly extend an arrangement of the at least one first and secondshim magnets 1220, and the first and second magnetically permeablematerial 1210 in a symmetrical fashion along a longitudinal axis 560 ofthe tool.

The first and second shim magnets and/or the first and secondmagnetically permeable material can be assembled with relative sizedifferences to enable different depths of investigation. Thus, in someembodiments, geological formation depths of inspection associated withthe first shim magnet 1320 and the second shim magnet 1380 aredifferent.

In most embodiments, the central and end piece magnets define magneticfields that have an orthogonal orientation. Thus, in some embodiments,the central magnet 510 defines a first magnetic field orientation, andthe first and second end piece magnets 520, 530 each define a secondmagnetic field orientation that is substantially orthogonal to the firstmagnetic field orientation.

The central and end piece magnets may comprise permanent magnets,electromagnets, and other kinds of magnets. Thus, in some embodiments,at least one of the central magnet 510 or the first and second end piecemagnets 520, 530 comprise one or more permanent magnets.

The apparatus 2110 may include one or more antennas that can be used todetect an NMR response from one or more magnetic fields defined by themagnets. Thus, in some embodiments, the central magnet 510 and the firstand second end piece magnets 520, 530 define an overall static magneticfield volume that comprises multiple distinct sub-volumes 1502, 1504,1506, the multiple distinct sub-volumes 1502, 1504, 1506 including thefirst static magnetic field sub-volume 1506 that is elongate in a firstdirection parallel to a longitudinal axis of the tool 560, the magneticfield in the first static magnetic field sub-volume 1506 beingsubstantially uniformly oriented in the first direction, the apparatusfurther comprising: antennas 35, 36, 37A, 37B located at respectivelocations along the longitudinal axis 560, each of the antennas 35, 36,37A, 37B to detect a nuclear magnetic resonance response from arespective one of the distinct sub-volumes 1502, 1504, 1506 when theantennas 35, 36, 37A, 37B are operating.

The magnetically permeable material may be selected to have a relativelylow conductivity, so that losses in the material are less than those inthe antennas and the formation. Thus, in some embodiments, conductivityof the first magnetically permeable material 550 is selected to reducelosses in the magnetically permeable material 550 to less than expectedlosses in the antennas 35, 36, 37A, 37B or the geological formation tobe investigated.

Antennas in the apparatus 2110 may comprise a variety of types, such asone or more transversal-dipole antennas to produce circular-polarizedexcitation in the magnetic field volume, or perhaps to acquire aresponse from the magnetic field volume by quadrature coil detection.Copper can be disposed over the central magnet, with magneticallypermeable material over the copper, and under the antenna(s). Thus, insome embodiments, the apparatus 2110 further comprises an antennaassembly 35, 36 comprising a transversal-dipole antenna 35 disposed overthe central magnet 510, with copper 1570 and central magneticallypermeable material 550 disposed between the central magnet 510 and theantenna assembly 35, 36.

In some embodiments, an apparatus 2110 comprises a downhole tool 102that is attached to a magnet assembly that includes a central magnet 510in-between two end piece magnets 520, 530, with magnetically permeablematerial 1210 located in-between the central magnet 510 and a first endof one of the end piece magnets 520, 530, and one or more shim magnets1560 located next to a second end of one of the end piece magnets 520,530 (e.g., see FIG. 20). Therefore, in some embodiments, an apparatus2110 comprises a magnet assembly to produce a magnetic field 1506 in avolume in a subterranean region, the magnet assembly comprising acentral magnet 510 having a first axial end and a second, opposite axialend; a first end piece magnet 520 having a proximal end and a distalend, the proximal end spaced apart from the first axial end of thecentral magnet 510; and a second end piece magnet 530 spaced apart fromthe second axial end of the central magnet 510; at least one first shimmagnet 1560 spaced apart from the distal end of the first end piecemagnet 520. The apparatus 2110 further comprises a downhole tool 102attached to the magnet assembly, further comprising first magneticallypermeable material 1550 disposed adjacent to or at least partiallysurrounding a longitudinal axis 560 of the tool, the first magneticallypermeable material 1550 disposed between the first end piece magnet 520and the first shim magnet 1560.

The magnetically permeable material can be used to surround additionalshim magnets. Thus, in some embodiments, the apparatus 2110 furthercomprises at least one second shim magnet 1560 disposed adjacent to orat least partially surrounded by the first magnetically permeablematerial 1550.

Copper may be placed between the magnetically permeable material andadditional shim magnets. Thus, in some embodiments, the apparatus 2110further comprises copper 1570 disposed between the magneticallypermeable material and the at least one second shim magnet.

The apparatus 2110 may include a symmetrical, mirrored arrangement ofadditional magnetic and magnetically permeable elements. Thus, in someembodiments, the apparatus 2110 further comprises at least one secondshim magnet 1560 spaced apart from a distal end of the second end piecemagnet 530 and second magnetically permeable material 1550 disposedbetween the second end piece magnet 530 and the second shim magnet 1560,to mirror an arrangement of the first shim magnet 1560 and the firstmagnetically permeable material 1550 about a center point 2010 of thecentral magnet 510.

The apparatus 2110 may include an asymmetric arrangement of additionalmagnetic and magnetically permeable elements. Thus, in some embodiments,the apparatus 2110 further comprises at least one second shim magnet1220 disposed between the central magnet 510 and the second end piecemagnet 530 and second magnetically permeable material 1210 disposedadjacent to or at least partially surrounding the at least one secondshim magnet 1220.

In some embodiments, a system 2100 comprises a tool 102 that comprisesmagnetically permeable material 1310, 1370, and shim magnets 1320located inside and/or outside of the junction between the central magnet510 and end piece magnets 520, 530 (e.g., see FIG. 13). The tool 102 iscoupled to a transmitter and receiver (e.g., transceiver 2104) to exciteand receive an NMR response in a geological formation. Thus, in someembodiments, a system 2100 comprises a magnet assembly to produce amagnetic field in a volume in a subterranean region, the magnet assemblycomprising any or all of the components shown in FIGS. 200A, 200B, 300B,FIGS. 12-13, FIGS. 15-16, FIG. 18, and/or FIG. 20. This includes acentral magnet 510 having a first axial end and a second, opposite axialend; a first end piece magnet 520 having a proximal end and a distalend, the proximal end spaced apart from the first axial end of thecentral magnet 510; and a second end piece magnet 530 spaced apart fromthe second axial end of the central magnet 510. In some embodiments, thesystem 2100 further comprises at least one first shim magnet 1220, 1660disposed adjacent to or at least partially surrounded by firstmagnetically permeable material 1210, 1550, the at least one first shimmagnet 1220, 1660 disposed next to an end of the first end piece magnet520 that is proximal to the central magnet 510 (e.g., as is the case formagnet 1220), or next to an end of the first end piece magnet 520 thatis distal to the central magnet 510 (e.g., as is the case for magnet1660). The system 2100 further comprises a downhole tool 102 attached tothe magnet assembly, further comprising a transmitter and a receiver(e.g., as the transceiver 2104) to excite and receive an NMR response inthe magnetic field volume.

The tool may comprise a wireline or drilling tool. Thus, in someembodiments, of the system 2100, the downhole tool 102 comprises one ofa wireline tool or a drilling tool (e.g., see FIGS. 1A, 1B, 1C).

Magnetic shims may be added to any of the configurations, and sized toprovide similar or different DOIs. Thus, in some embodiments, a system2100 comprises at least one second shim magnet 1660″ adjacent to or atleast partially surrounded by second magnetically permeable material1550, the at least one second shim magnet 1660″ disposed next to an endof the second end piece magnet 530 that is proximal to the centralmagnet 510, or next to an end of the second end piece magnet 530 that isdistal to the central magnet 510, wherein geological formation depths ofinspection associated with the first shim magnet 1660′ and the secondshim magnet 1660″ are different.

One or more antennas may be added to the basic configuration. Thus, insome embodiments, a system 2100 further comprises multiple antennas 35,36, 37A, 37B disposed at respective locations along a longitudinal axisof the tool 560, each one of the antennas 35, 36, 37A, 37B to detect anuclear magnetic response from a respective one of multiplecorresponding distinct magnetic field sub-volumes defined by the centraland first and second end piece magnets 510, 520, 530. Still furtherembodiments may be realized.

Additional Methods

In some embodiments, a non-transitory machine-readable storage devicecomprises instructions stored thereon, which, when executed by amachine, transforms the machine into a customized, particular machinethat performs operations comprising one or more features similar to oridentical to those described with respect to the methods and techniquesdescribed herein. A machine-readable storage device, as describedherein, is a physical device that stores information (e.g.,instructions, data), which when stored, alters the physical structure ofthe device. Examples of machine-readable storage devices can include,but are not limited to, memory 2106 in the form of read only memory(ROM), random access memory (RAM), a magnetic disk storage device, anoptical storage device, a flash memory, and other electronic, magnetic,or optical memory devices, including combinations thereof.

The physical structure of stored instructions may be operated on by oneor more processors such as, for example, the processing unit 2102.Operating on these physical structures can cause the machine to become aspecialized machine that performs operations according to methodsdescribed herein. The instructions can include instructions to cause theprocessing unit 2102 to store associated data or other data in thememory 2106. The memory 2106 can store the results of NMR measurementsof formation parameters, drilling operation parameters, gain parameters,calibration constants, identification data, sensor location information,etc. The memory 2106 can store a log of the measurement and locationinformation provided by the system 2100. The memory 2106 therefore mayinclude a database, for example a relational database.

The apparatus 2110 and system 2100, and each of their elements may allbe characterized as “modules” herein. Such modules may include hardwarecircuitry, and/or a processor and/or memory circuits, software programmodules and objects, and/or firmware, and combinations thereof, asdesired by the architect of the apparatus 2110 and system 2100, and asappropriate for particular implementations of various embodiments. Forexample, in some embodiments, such modules may be included in anapparatus and/or system operation simulation package, such as a softwareelectrical signal simulation package, a power usage and distributionsimulation package, a power/heat dissipation simulation package, aformation imaging package, an energy detection and measurement package,and/or a combination of software and hardware used to simulate theoperation of various potential embodiments.

It should also be understood that the apparatus and systems of variousembodiments can be used in applications other than for loggingoperations, and thus, various embodiments are not to be so limited. Theillustrations of apparatus 2110 and system 2100 are intended to providea general understanding of the structure of various embodiments, andthey are not intended to serve as a complete description of all theelements and features of apparatus and systems that might make use ofthe structures described herein.

Applications that may include the novel apparatus and systems of variousembodiments include electronic circuitry used in high-speed computers,communication and signal processing circuitry, modems, processormodules, embedded processors, data switches, and application-specificmodules. Such apparatus and systems may further be included assub-components within a variety of electronic systems, such astelevisions, cellular telephones, personal computers, workstations,radios, vehicles, geothermal tools, NMR imaging systems, and smarttransducer interface node telemetry systems, among others. Someembodiments include a number of methods.

For example, FIG. 22 is a flow chart illustrating several methods 2211according to various embodiments. The methods 2211 may compriseprocessor-implemented methods, to execute on one or more processors thatperform the methods. For example, one embodiment of the methods 2211 maybegin at block 2221 with deploying a magnet assembly, as part of anapparatus, downhole. The apparatus may comprise any form of theapparatus 2110 illustrated in FIG. 21, for example, such as a wirelinetool, or a drilling tool, etc. The method 2211 may continue on to block2225 with the acquisition of NMR data, using the apparatus deployed in2221. In some embodiments, the method 2211 comprises the activation ofsymmetric zones of sensitivity at block 2229. In some embodiments, themethod 2211 comprises the activation of asymmetric zones of sensitivityat block 2229. Whether the zones are symmetric or asymmetric isdetermined by the configuration of the magnet assembly on the apparatusdeployed in block 2221. The method 2211 may continue on to repeat theactions at blocks 2221, 2225, and/or 2229.

It should be noted that the methods described herein do not have to beexecuted in the order described, or in any particular order. Moreover,various activities described with respect to the methods identifiedherein can be executed in iterative, serial, or parallel fashion. Thevarious elements of each method (e.g., the methods shown in FIGS. 4A,4B, and 22) can be substituted, one for another, within and betweenmethods. Information, including parameters, commands, operands, andother data, can be sent and received in the form of one or more carrierwaves.

Upon reading and comprehending the content of this disclosure, one ofordinary skill in the art will understand the manner in which a softwareprogram can be launched from a computer-readable medium in acomputer-based system to execute the functions defined in the softwareprogram. One of ordinary skill in the art will further understand thevarious programming languages that may be employed to create one or moresoftware programs designed to implement and perform the methodsdisclosed herein.

For example, the programs may be structured in an object-orientatedformat using an object-oriented language such as Java or C#. In anotherexample, the programs can be structured in a procedure-orientated formatusing a procedural language, such as assembly or C. The softwarecomponents may communicate using any of a number of mechanisms wellknown to those skilled in the art, such as application programinterfaces or interprocess communication techniques, including remoteprocedure calls. The teachings of various embodiments are not limited toany particular programming language or environment.

In summary, the apparatus, systems, and methods disclosed herein makeuse of additional shim magnets and magnetically permeable material toshape the NMR magnetic field to provide a variable DOI, with improvedSNR. The resulting flexibility in tool designed may significantlyenhance the value of services provided by an operation/explorationcompany.

The accompanying drawings that form a part hereof, show by way ofillustration, and not of limitation, specific embodiments in which thesubject matter may be practiced. The embodiments illustrated aredescribed in sufficient detail to enable those skilled in the art topractice the teachings disclosed herein. Other embodiments may beutilized and derived therefrom, such that structural and logicalsubstitutions and changes may be made without departing from the scopeof this disclosure. This Detailed Description, therefore, is not to betaken in a limiting sense, and the scope of various embodiments isdefined only by the appended claims, along with the full range ofequivalents to which such claims are entitled.

The Abstract of the Disclosure is provided to quickly ascertain thenature of the technical disclosure. It is submitted with theunderstanding that it will not be used to interpret or limit the scopeor meaning of the claims. In addition, in the foregoing DetailedDescription, it can be seen that various features are grouped togetherin a single embodiment for the purpose of streamlining the disclosure.This method of disclosure is not to be interpreted as reflecting anintention that the claimed embodiments require more features than areexpressly recited in each claim. Rather, as the following claimsreflect, inventive subject matter lies in less than all features of asingle disclosed embodiment. Thus the following claims are herebyincorporated into the Detailed Description, with each claim standing onits own as a separate embodiment.

1. A method comprising: disposing a downhole tool having a magnetassembly into a wellbore; generating, using the magnet assembly, amagnetic polarization in a volume into a subterranean region about thewellbore; emitting an excitation in the magnetic polarization in thevolume in the subterranean region; detecting, by at least one antenna, anuclear magnetic resonance response to the excitation of the volume inthe subterranean region; and determining a property of the subterraneanregion based on the nuclear magnetic resonance response.
 2. The methodof claim 1, wherein generating the magnetic polarization comprisesgenerating a magnetic polarization of nuclear spins.
 3. The method ofclaim 1, wherein detecting the nuclear magnetic resonance responsecomprises detecting an azimuthally-selective nuclear magnetic resonanceresponse.
 4. The method of claim 3, wherein the at least one antennacomprises multiple antennas disposed at respective locations along alongitudinal axis of the downhole tool.
 5. The method of claim 4,wherein the magnet assembly comprises a central magnet having a firstaxial end and a second axial end that is opposite the first axial end, afirst end piece magnet spaced apart from the first axial end of thecentral magnet, a second end piece magnet spaced apart from the secondaxial end of the central magnet, and at least one first shim magnetdisposed between the central magnet and the first end piece magnet, thefirst shim magnet disposed adjacent to or at least partially surroundedby a first magnetically permeable material, the first shim magnet toshape a first static magnetic field sub-volume provided by the centralmagnet and the first end piece magnet.
 6. The method of claim 5, whereindetecting, by the at least one antenna, of the nuclear magneticresonance response comprises detecting, by each of the multiple antennasthe nuclear magnetic resonance response from a respective one ofmultiple corresponding distinct magnetic field sub-volumes defined bythe central and first and second end piece magnets.
 7. The method ofclaim 5, wherein the magnet assembly comprises a second shim magnetdisposed between the central magnet and the second end piece magnet, thesecond shim magnet disposed adjacent a second magnetically permeablematerial or at least partially surrounded by the second magneticallypermeable material and a copper material.
 8. The method of claim 1,further comprising controlling a drilling operation based on theproperty of the subterranean region.
 9. A method comprising: disposing adownhole tool into a wellbore, wherein the downhole tool comprises amagnet assembly that comprises a central magnet having a first axial endand a second axial end that is opposite the first axial end, a first endpiece magnet spaced apart from the first axial end of the centralmagnet, a second end piece magnet spaced apart from the second axial endof the central magnet, and at least one first shim magnet disposedbetween the central magnet and the first end piece magnet, the firstshim magnet disposed adjacent to or at least partially surrounded by afirst magnetically permeable material, the first shim magnet to shape afirst static magnetic field sub-volume provided by the central magnetand the first end piece magnet; outputting, using the magnet assembly, amagnetic field in a volume into a subterranean region around thewellbore; emitting an excitation into the volume of the magnetic fieldin the subterranean region; detecting, by at least one antenna, anuclear magnetic resonance response to the excitation of the volume ofthe magnetic field; and determining a property of the subterraneanregion based on the nuclear magnetic resonance response.
 10. The methodof claim 9, wherein the at least one antenna comprises multiple antennasdisposed at respective locations along a longitudinal axis of thedownhole tool.
 11. The method of claim 10, wherein detecting, by the atleast one antenna, of the nuclear magnetic resonance response comprisesdetecting, by each of the multiple antennas the nuclear magneticresonance response from a respective one of multiple correspondingdistinct magnetic field sub-volumes defined by the central and first andsecond end piece magnets.
 12. The method of claim 9, wherein outputtingthe magnetic field comprises generating a magnetic polarization ofnuclear spins.
 13. The method of claim 9, wherein detecting the nuclearmagnetic resonance response comprises detecting an azimuthally-selectivenuclear magnetic resonance response.
 14. The method of claim 9, furthercomprising controlling a drilling operation based on the property of thesubterranean region.
 15. A system, comprising: a magnet assembly toproduce a magnetic field in a volume in a subterranean region, themagnet assembly comprising: a central magnet having a first axial endand a second axial end that is opposite the first axial end; a first endpiece magnet having a proximal end and a distal end, the proximal endspaced apart from the first axial end of the central magnet; a secondend piece magnet spaced apart from the second axial end of the centralmagnet; and at least one first shim magnet disposed adjacent to or atleast partially surrounded by first magnetically permeable material theat least one first shim magnet disposed next to an end of the first endpiece magnet that is proximal to the central magnet or next to an end ofthe first end piece magnet that is distal to the central magnet; atleast one antenna that includes, a transmitter to emit an excitationinto the volume of the magnetic field in the subterranean region; and areceiver to detect a nuclear magnetic resonance response in the volumeof the magnetic field; a processor; and a machine-readable medium havingprogram code executable by the processor to cause the processor todetermine a property of the subterranean region based on the nuclearmagnetic resonance response.
 16. The system of claim 15, wherein the atleast one antenna comprises multiple antennas disposed at respectivelocations along a longitudinal axis of a downhole tool.
 17. The systemof claim 16, wherein the receiver in each of the multiple antennas is todetect the nuclear magnetic resonance response from a respective one ofmultiple corresponding distinct magnetic field sub-volumes defined bythe central and first and second end piece magnets.
 18. The system ofclaim 15, wherein the magnetic field comprises a magnetic polarizationof nuclear spins.
 19. The system of claim 15, wherein the nuclearmagnetic resonance response comprises an azimuthally-selective nuclearmagnetic resonance response.
 20. The system of claim 15, wherein theprogram code comprises program code executable by the processor to causethe processor to control a drilling operation based on the property ofthe subterranean region.